The present invention is in the field of reinforced ceramic composites. More particularly, the invention relates to whisker-reinforced ceramic matrix composites comprising glass-ceramic matrices and being formed by a direct extrusion process.
The use of inorganic whiskers and fibers to reinforce glasses, glass-ceramics and ceramics has long been practiced. The literature frequently refers to whiskers as relatively short, single-crystal fibers of small diameter (less than 100 microns), while fibers are considered to be multicrystalline or amorphous and are generally sufficiently long to be used in woven or otherwise interlocking bundles, tows or cloth. Hence whiskers are typically incorporated as a randomly dispersed phase, while fibers are more frequently incorporated in a controlled oriented or interlocking alignment.
The mechanism of strengthening of glass or ceramic bodies by fibers is considered to be that of load transfer by the matrix to the fibers in shear. This load transfer shifts stress from the glass or ceramic matrix to the relatively long, high modulus fibers, while the fibers at the same time may act to impede crack propagation through the matrix material.
Whiskers are thought to impart strengthening by similar mechanism, but load transfer to whiskers by the matrix is more limited due to the limited length and aspect ratio of the whiskers. Theoretically, a whisker which is sufficiently short will not be loaded to the breaking point by the matrix under stress, and therefore full advantage cannot be taken of the high strength of the whiskers.
Among the fibers and whiskers which have been suggested for use as reinforcement for nonmetal matrix materials are whiskers and/or fibers of silicon carbide, silicon nitride, alumina, glass, and carbon. Ceramic matrix materials which have been prepared for use in composites have included glasses, glass-ceramics, and conventional ceramics.
U.S. Pat. No. 4,324,843 describes a family of fiber-reinforced glass-ceramic composite bodies wherein the glass-ceramic matrix is of aluminosilicate composition and the reinforcing fibers consist of silicon carbide. U.S. Pat. No. 4,464,475 describes similarly reinforced glass-ceramics comprising barium osumilite as the predominant crystal phase, while U.S. Pat. No. 4,464,192 describes whisker-reinforced glass-ceramic composites of aluminosilicate composition.
U.S. Pat. No. 4,615,987 discloses composite whisker-reinforced glass-ceramic products wherein anorthite and/or barium-stuffed cordierite crystals are developed in the matrix, while U.S. Pat. No. 4,588,699 discloses fiber composites which include a barium-stuffed cordierite glass-ceramic matrix. In these cases, however, a hot-pressing consolidation process is used to achieve strong, dense products.
Glass-ceramics represent a particularly promising group of matrix materials for reinforced ceramics because of the processing advantages theoretically obtainable therewith. Hence, since they are derived by thermal crystallization of glasses of appropriate composition, composite products can be consolidated under the relatively mild conditions appropriate for the consolidation of glasses. Yet the glasses may be thermally crystallized during or subsequent to consolidation to crystalline materials, and if crystallization is complete and residual glassy phases eliminated, phase assemblages highly resistant to creep or dimensional change at elevated temperatures may be developed in the composite matrix.
The preferred forming practice for the manufacture of ceramic matrix composites generally involves consolidation under pressure at relatively high temperatures. Even where glasses are employed as the matrix material, the glass-whisker or glass-fiber mixtures making up preforms for such composites generally exhibit rather high effective viscosities even at elevated processing temperatures, due to the presence of whiskers and/or fibers therein. Thus procedures such as hot-pressing or hot isostatic pressing are generally required to minimize the presence of defects or voids in the composite matrix.
Of course, the need to process composite preforms at high temperatures and at superatmospheric pressures adds complexity and cost to the manufacturing operation. But since the presence of voids has been deemed unacceptable from the standpoint of achieving the desired strength and toughness in the composite material, such costs have been assumed as a necessary part of the production of these materials.
While extrusion processing seems attractive as an approach to the manufacture of complex shapes in whisker- or fiber-reinforced materials, this approach has not been utilized to any degree, either commercially or in development, because the consolidation of extruded preforms of complex shape cannot be conveniently accomplished under high-pressure conditions. Hence conventional hot-pressing cannot be used to consolidate extruded products such as tubular or honeycomb structures.
It is therefore a principal object of the present invention to provide an extrusion process for the production of whisker-reinforced ceramic products which yields articles of excellent toughness and high modulus of rupture strength without the need to utilize a pressure consolidation process.
It is a further object of the invention to provide a method for making whisker-reinforced ceramic matrix composites which, while in some cases yielding a product with some residual porosity, still provides the necessary strength and toughness for high-temperature structural applications.
It is a further object of the invention to provide a whisker-reinforced ceramic matrix composite article exhibiting unexpected strength and toughness for structural or other high-temperature applications.
It is a further object of the present invention to provide a method for extruding whisker-reinforced ceramic matrix composites which can yield substantially void-free composite ceramic articles.
Other objects and advantages of the invention will become apparent from the following description thereof.